Because of its low melting point and lack of toxicity, gallium has been used in many diverse applications. For example, gallium has been used instead of mercury in dental alloys, and has been used as a heat transfer medium. Also, gallium has found limited use as the thermometric fluid for high temperature thermometers. More recently high purity gallium has been alloyed with phosphorus or arsenic to form gallium phosphide or gallium arsenide which have found extensive use in the electronics industry.
No ore is known presently which contains any appreciable concentration of gallium. The element is very widely distributed, particularly in aluminous materials where the gallium content is roughly proportional to the aluminum content. Gallium is also found in small concentrations in most zinc blendes, and in germanite where it appears with the mixed sulfides.
Because of the variety of minerals in which gallium occurs, many processes have been developed to concentrate the element. The concentration of gallium from germanite, the richest natural source of gallium, involves the conversion of the germanium and gallium to the chloride, and the more volatile germanium tetrachloride is separated by distillation while the gallium is recovered by electrolysis.
In one of the prior art industrial processes for the recovery of gallium, carbon dioxide gas is blown through a filtrate of sodium aluminate solution obtained as a by-product in the course of the manufacture of alumina to precipitate crude hydroxide of gallium. Alternatively, the filtrate is electrolyzed by using a mercury cathode to produce a gallium amalgam.
In another process for the recovery of gallium, a residue from an electrolytic process or pyrometallurgical process is utilized. The residue is subjected to an acid extraction to prepare a gallium-containing solution, which is then neutralized to precipitate a crude hydroxide of gallium.
The gallium hydroxide is dissolved in a hydrochloric acid solution and thereafter subjected to a liquid-liquid extraction with isopropyl ether to recover the gallium.
Gallium may also be recovered from the alumina purification processes of the aluminum industry. In the Bayer process, aluminum trihydrate is crystallized from a solution of a sodium aluminate by cooling and seeding. In this process, gallium accumulates in the liquor. After concentration of the liquor and an adjustment of the pH, the gallium may be separated by electrolysis. On the other hand, U.S. Pat. No. 2,582,376 discloses a process for removing gallium from alkaline solutions containing dissolved alkali metal aluminate and gallium by adding a soluble calcium compound which results in the precipitation of calcium aluminate while leaving the gallium in the solution. The gallium can then be precipitated and the precipitate redissolved in a solvent to provide a concentrated solution of gallium. Metallic gallium can then be recovered by electrolysis.
The prior art techniques mentioned above have many disadvantages. For example, the process which utilizes blowing of carbon dioxide or neutralization to prepare hydroxide of gallium is complicated in its operations because filtration of the solution is very difficult. In addition, where a starting solution contains many kinds of elements, such as iron, copper, aluminum, etc., too many hydroxides are involved and the yield and selectivity of a desired metal becomes poor.
The electrolytic process utilizing a mercury cathode has disadvantages because current efficiency is very low. The process employing isopropyl ether as an extraction solvent can separate gallium in high selectivity, but it requires a highly concentrated hydrochloric acid solution, which has a high solubility for isopropyl ether, so that life of the solvent is relatively short.
It has now been found that by practice of the present invention, gallium may be recovered from a leach solution rich in chloride values by multiple selective extraction steps which isolate gallium from chloride solutions of aluminum and iron. Thus, difficulties and disadvantages of prior art attempts to recover gallium from leach solutions have been overcome in a simple, highly efficient manner.